Because the lead of the google photos team in charge of that enrollment funnel will get a pat on the head for having solid Artifacts detailing how many tens of thousands of users they got to sign up. I don't think there's any nefarious scheme, they just want you firmly and permanently locked into their ecosystem, and every little step helps.

Plus, it's one of those things where 90% of tech-illiterate people probably do want it. So it can make some benign sense to spam it a bit -- though that obviously sounds excessive.